.TH std::memcpy 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::memcpy \- std::memcpy

.SH Synopsis
   Defined in header <cstring>
   void* memcpy( void* dest, const void* src, std::size_t count );

   Copies count bytes from the object pointed to by src to the object pointed to by
   dest. Both objects are reinterpreted as arrays of unsigned char.

   If the objects overlap, the behavior is undefined.

   If either dest or src is an invalid or null pointer, the behavior is undefined, even
   if count is zero.

   If the objects are potentially-overlapping or not TriviallyCopyable, the behavior of
   memcpy is not specified and may be undefined.

.SH Parameters

   dest  - pointer to the memory location to copy to
   src   - pointer to the memory location to copy from
   count - number of bytes to copy

.SH Return value

   dest

.SH Notes

   std::memcpy may be used to implicitly create objects in the destination buffer.

   std::memcpy is meant to be the fastest library routine for memory-to-memory copy. It
   is usually more efficient than std::strcpy, which must scan the data it copies or
   std::memmove, which must take precautions to handle overlapping inputs.

   Several C++ compilers transform suitable memory-copying loops to std::memcpy calls.

   Where strict aliasing prohibits examining the same memory as values of two different
   types, std::memcpy may be used to convert the values.

.SH Example


// Run this code

 #include <cstdint>
 #include <cstring>
 #include <iostream>

 int main()
 {
     // simple usage
     char source[] = "once upon a daydream...", dest[4];
     std::memcpy(dest, source, sizeof dest);
     std::cout << "dest[4] = {";
     for (int n{}; char c : dest)
         std::cout << (n++ ? ", " : "") << '\\'' << c << "'";
     std::cout << "};\\n";

     // reinterpreting
     double d = 0.1;
 //  std::int64_t n = *reinterpret_cast<std::int64_t*>(&d); // aliasing violation
     std::int64_t n;
     std::memcpy(&n, &d, sizeof d); // OK

     std::cout << std::hexfloat << d << " is " << std::hex << n
               << " as a std::int64_t\\n" << std::dec;

     // object creation in destination buffer
     struct S
     {
         int x{42};
         void print() const { std::cout << '{' << x << "}\\n"; }
     } s;
     alignas(S) char buf[sizeof(S)];
     S* ps = new (buf) S; // placement new
     std::memcpy(ps, &s, sizeof s);
     ps->print();
 }

.SH Output:

 dest[4] = {'o', 'n', 'c', 'e'};
 0x1.999999999999ap-4 is 3fb999999999999a as a std::int64_t
 {42}

.SH See also

   memmove               moves one buffer to another
                         \fI(function)\fP
   memset                fills a buffer with a character
                         \fI(function)\fP
                         copies a certain amount of wide characters between two
   wmemcpy               non-overlapping arrays
                         \fI(function)\fP
                         copies characters
   copy                  \fI\fI(public member\fP function of\fP
                         std::basic_string<CharT,Traits,Allocator>)
   copy                  copies a range of elements to a new location
   copy_if               \fI(function template)\fP
   \fI(C++11)\fP
   copy_backward         copies a range of elements in backwards order
                         \fI(function template)\fP
   is_trivially_copyable checks if a type is trivially copyable
   \fI(C++11)\fP               \fI(class template)\fP
   C documentation for
   memcpy
